1. Field of the Invention
The present invention relates to an audio apparatus for outputting an audio signal, and, more particularly, to control of current consumption in an audio apparatus for use in a vehicle.
2. Description of the Related Art
In audio apparatuses installed in mobile units such as vehicles, the number of channels of an amplifier is increasing and the output power of the amplifier is therefore becoming higher. With the increasingly higher output power, a relatively large current is supplied to an audio apparatus. On the other hand, when a trouble such as a short circuit occurs in an audio apparatus, it is necessary to protect an electronic circuit from an overcurrent. Japanese Unexamined Patent Application Publication No. 2008-244554 discloses a technique for protecting a power amplifier from an overcurrent by determining the occurrence of an overcurrent due to a short circuit, activating a limiter circuit when determining that an overcurrent has occurred, limiting the amplitude of an audio signal input into a power amplifier at a stage subsequent to the limiter circuit, and reducing the amount of power consumption of the power amplifier.
Because of the increase in the output power of a power amplifier in an audio apparatus for use in a vehicle and the increase in the number of channels of the power amplifier, a large current may be consumed and a fuse may be blown in a case where rated power is output from all channels. In order to prevent the blowout of a fuse at the time of the output of big sound, it is necessary to limit current consumption. Under the present circumstances, a digital signal processor (DSP) for performing signal processing for an audio apparatus reduces power consumption by estimating the value of an output current and decreasing the threshold value (amplitude) of a limiter so as to prevent an output current from exceeding a predetermined value or by measuring a voltage across a small resistor inserted into a power-supply line to measure a current and decreasing the threshold value (amplitude level) of a limiter at the time of occurrence of a large current.
FIG. 1 illustrates a waveform H1 of a voltage supplied from a battery, a waveform H2 of a consumption current of a power amplifier, and an output waveform H3 of a speaker. In general, a DC voltage of 12 V is supplied from a battery to an audio apparatus, and the waveform of the supplied voltage is represented by H1. In a case where audio signals having rated power are output from all channels of a power amplifier in an audio apparatus, current consumption markedly increases as represented by, for example, the waveform H2 at a time T1. Since a large current flows at that time, voltage from the battery temporarily decreases. From the fact that audio signals are output from all channels of a power amplifier or a voltage drop exceeding a predetermined level occurs at a power-supply line from the battery, it is estimated that current consumption exceeds a predetermined value at the time T1 as described above. The levels of the audio signals are therefore reduced to a predetermined level by a limiter. In the example illustrated in FIG. 1, since it is estimated that relatively large power is consumed in a period from the time T1 to a time T2, the amplitude of the voltage waveform H3 representing the output of a speaker is clamped at a predetermined level. After the time T2, a limiter is deactivated and the voltage waveform H3 has no clamped portion.
However, in the case of the control of power consumption in an audio apparatus in the related art, the blowout of a fuse does not occur but sound is sometimes stopped because of a voltage drop at a power-supply line. The length of a power-supply line from the battery to an amplifier in a vehicle may be 2 m or more. When a large current flows through this power-supply line, a voltage drop occurs. The power supply voltage of the amplifier therefore falls below an operating voltage and sound is stopped.
Furthermore, in a case where the control speed of a limiter is low, the blowout of a fuse occurs. It is desired that the threshold value (amplitude level) of a limiter be rapidly decreased to reduce the output of an amplifier in a case where the flow of a large current is detected. However, a limiter is also usually used to reduce a distortion. An attenuation speed tends to be set to a slow speed from the viewpoint of sound tuning, because sound quality is reduced in a case where an attenuation speed is markedly increased during a normal operation. However, the setting of a slow attenuation speed may lead to a delay in performing current control and cause the blowout of a fuse. On the other hand, the occurrence of blowout of a fuse can be prevented by increasing the rated value of the fuse. However, in a case where the rated value of a fuse is increased, a large current flows in the event of trouble and may cause a fire. It is therefore difficult to increase the rated value of a fuse more than necessary. Furthermore, in order to increase the rated value of a fuse, it is necessary to increase the thickness of a power-supply line.